Fix GET_PART_INFO/GET_SYS_STATE/GET_FUNC_INFO byte offsets; confirm Si4684 blobs genuine

Si4684Driver::getPartInfo()/getSysState() existed but were never called, so
their byte-offset bugs never surfaced. Wired them into a boot-time diagnostic
log (part number, firmware revision, active image, streamed blob byte counts)
to check whether the loaded DAB/FM firmware images are genuine and intact, as
an alternative explanation to the QFN exposed-pad hardware hypothesis.

Fixed two rounds of off-by-one bugs found while doing this: the fields were
initially read one byte too far right (e.g. firmwareBuild was reading a flag
byte, not a version number); the first fix undershot in the other direction
by not accounting for readRaw()'s one-byte SPI lead-in before STATUS0 (already
documented and confirmed elsewhere in this file, in pollStc()) -- caught
because the "fixed" GET_SYS_STATE reported image=192 (0xC0), the exact
signature of STATUS3 with PUP_STATE=3 seen throughout this investigation.
All three response buffers were already sized for the lead-in byte, which
confirmed the correct fix.

Verdict, captured live: blob streamed bytes match local file sizes exactly (no
truncation), GET_SYS_STATE reports image=2 (DAB active, correct), GET_PART_INFO
reports part=4684 (matches expected Si4684 part number) with a plausible
firmware revision -- the loaded DAB firmware is genuine and intact. This closes
the last plausible firmware-side explanation for the FM/DAB no-lock symptom;
docs/si4684-rf-investigation-report.md and docs/TODO.md (P4/H5) updated with
the full record and verdict.

Also noted, not yet fixed: BT1035 AT-init now fails deterministically on every
boot (was a one-off earlier this session) -- see report's Open Items.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Software/components/drivers/si4684/include/si4684/Si4684Types.hpp

@@ -62,9 +62,10 @@ enum class SeekBandWrap { Wrap, NoWrap };
  */
 struct Si4684PartInfo {
     std::uint16_t chipId;         ///< Part identifier from the chip.
-    std::uint8_t firmwareMajor;   ///< Loaded firmware major version.
-    std::uint8_t firmwareMinor;   ///< Loaded firmware minor version.
-    std::uint8_t firmwareBuild;   ///< Loaded firmware build number.
+    std::uint8_t firmwareMajor;   ///< Loaded function image major (REVEXT).
+    std::uint8_t firmwareMinor;   ///< Loaded function image minor (REVBRANCH).
+    std::uint8_t firmwareBuild;   ///< Loaded function image build (REVINT).
+    std::uint32_t svnId;          ///< GET_FUNC_INFO SVNID[31:0] build source.
 };
 
 /**